A method for purifying isopulegol is disclosed, comprising deep cooling
isopulegol mainly composed of (-)-n-isopulegol in a solvent for deep
cooling mainly comprising acetone to obtain (-)-n-isopulegol having a
chemical purity of not less than 99.7% by weight and an optical purity of
not less than 99.7%e.e. The thus purified (-)-n-isopulegol is odorless and
gives a feeling of freshness, crispness and coolness to a citrus perfume
composition.

1. A citrus perfume composition comprising citrus perfume components which
are blended with odorless (-)-n-isopulegol, said citrus perfume
composition containing (-)-n-isopulegol having a chemical purity of not
less than 99.7% by weight and an optical purity of not less than 99.7%
e.e. prior to use in said perfume composition and in which the
(-)-n-isopulegol component of the citrus perfume composition is not less
than 99.7 (-)-n-isopulegol by weight based on isopulegol and its geometric
and optical isomers and having an optical purity of not less than 99.7%
e.e. which is obtained by deep cooling isopulegol mainly composed of
(-)-n-isopulegol in a solvent for deep cooling mainly comprising acetone.

2. A citrus perfume composition containing said odorless (-)-n-isopulegol
having a chemical purity of not less than 99.7% by weight and an optical
purity of not less than 99.7%e.e. which is obtained by deep cooling
isopulegol mainly composed of (-)-n-isopulegol in a solvent for deep
cooling mainly comprising acetone, wherein (-)-n-isopulegol is present in
an amount from 3 to 30% by weight.

Description

FIELD OF THE INVENTION

This invention relates to a purification method for obtaining odorless and
chemically and optically pure (-)-n-isopulegol which gives a feeling of
freshness, crispness, and coolness to the fragrance of a perfume
composition and to a citrus perfume composition containing the thus
purified (-)-n-isopulegol.

BACKGROUND OF THE INVENTION

Industrial preparation of isopulegol mainly composed of the (-)-n-compound
comprises cyclization of (+)- citronellal. (+)-Citronellal (optical
purity: 80 to 85%e.e.) obtained from citronella oil has conventionally
been used as the starting material but recently been being replaced with
optically purer (+)-citronellal (optical purity: 97.5%e.e.; see Indo
Motoichi, Koryo, No. 177, pp 33-47 (1993), hereinafter referred to as
"Takasago Process") obtained by asymmetric isomerization of geranyl
diethylamine using an Rh-BINAP complex catalyst (Rh complex having
2,2'-bis(diphenylphosphino)-1,1'-binaphthyl as a ligand).

Isopulegol mainly composed of the (-)-n-compound (1- isopulegol) has a
minty, herbaceous and bitter sweet odor and gives off a bitter but sharp
note at a concentration of 50 ppm or higher and a herbaceous bitter note
or a bitter sweet and minty note at a concentration of about 10 ppm. In
the field of perfumery, it has been added to a perfume composition in a
small amount for the purpose of lifting a rose note, a geranium note, a
reseda note, an oriental note, a tuberose note, etc. (S. Arctander,
Perfume and Flavor Chemicals, Compound No. 2768). However, there is no
report of use of isopulegol in citrus perfume compositions because the
fragrance of conventionally available isopulegol does not agree with a
citrus note, and addition of isopulegol causes loss of the citrus note of
a citrus perfume composition.

With respect to a feeling of coolness of (-)-n- isopulegol, H. Yamazaki, et
al. report in Korvo, No. 95, pp. 39-43 (1970) that (-)-n-isopulegol
slightly gives a feeling of coolness, though making no mention of
fragrance, chemical purity or optical purity. An unexamined published
Japanese patent application (JP-A) No. 6-65023) discloses applicability of
(-)-n-isopulegol as an agent for giving a feeling of coolness. However, no
mention of the fragrance of (-)-n-isopulegol still less use of
(-)-n-isopulegol in citrus perfume compositions is found in these reports.

With the recent tendency to diversity of perfumery products, development of
new perfume material excellent in various physical properties, such as
safety, stability, preference, freshness, and a natural feeling, as well
as quality of odor. A citrus note is one type of odors which meets all the
public taste irrespective of sex and age and has been used widely, and it
has been demanded to develop a compound which further refines a citrus
fragrance by adding a feeling of freshness, crispness and coolness to
create a more natural and more preferred fragrance.

SUMMARY OF THE INVENTION

The inventor of the present invention has conducted extensive
investigations and found as a result that optically and chemically pure
(-)-n-isopulegol is odorless and that the optically and chemically pure
(-)-n-isopulegol imparts a feeling of freshness, crispness, and coolness
to a citrus note.

The present invention relates to a method for purifying isopulegol
comprising deep cooling isopulegol mainly composed of (-)-n-isopulegol in
a deep cooling solvent mainly comprising acetone to obtain
(-)-n-isopulegol having a chemical purity of not less than 99.7% by weight
and an optical purity of not less than 99.7%e.e.

The present invention further relates to a citrus perfume composition
containing the (-)-n-isopulegol obtained by the above purification method,
preferably in an amount of from 3 to 30% by weight.

DETAILED DESCRIPTION OF THE INVENTION

Isopulegol has three asymmetric carbon atoms and includes four geometric
isomers (n-form, neo-form, iso-form, and neoiso-form) and eight optical
isomers. The eight optical isomers are shown below.
##STR1##

As previously described, isopulegol that has conventionally been used as a
perfume component is a mixture of the eight optical isomers mainly
comprising (-)-n-isopulegol which is synthesized by cyclization of
(+)-citronellal obtained from citronella oil and having an optical purity
of 80 to 85%e.e. (d-form: 90 to 92.5%; l-form: 10 to 7.5%) or
(+)-citronellal obtained by asymmetric isomerization of geranyl
diethylamine using an Rh-BINAP catalyst and having an optical purity of
97.5 to 98%e.e. (d- form: 98.75 to 99%; l-form: 1.25 to 1.0%).

In menthol synthesis according to the above Takasago Process,
(-)-n-isopulegol having a chemical purity of 97.1% by weight and an
optical purity of 97.5%e.e. is obtained as an intermediate. This compound
can be purified by deep cooling in a petroleum hydrocarbon, followed by
recrystallization once or twice, and followed by distillation to give
(-)-n-isopulegol having an optical purity of not lower than 99%e.e. and a
chemical purity of 99 to 99.7% by weight. The thus purified compound still
has a relatively mild, minty herbaceous, and bitter sweet scent and can be
used as a perfume component.

There are reports on optically pure (-)-n-isopulegol obtained by, for
example, repeated recrystallization in petroleum ether (Perfume &
Essential O.R., p. 365 (1968)) or recrystallization of a magnesium salt of
isopulegol phthalate (J. Chem. Soc., p. 1248 (1920)), but no mention of
the fragrance of the thus purified compound is made in the reports.

Assuming that the smell of (-)-n-isopulegol is ascribed to less than 0.3%
by weight of impurity, the inventor aimed at examining the fragrance of
(-)-n-isopulegol having a chemical and optical purity of 99.7% by weight
or higher. For this purpose, the inventor purified the above-mentioned
(-)-n-isopulegol having an optical purity of not lower than 99%e.e. and a
chemical purity of 99 to 99.7% by weight by cooling at -30.degree. to
-35.degree. C. while stirring in twice (vol/wt) as much petroleum ether as
the isopulegol, separating the precipitated crystals by centrifugation,
and repeating the same operation (deep cooling) six times to obtain
(-)-n-isopulegol having a chemical purity of 100% by weight and an optical
purity of 100%e.e. (›.alpha.!.sub.D.sup.25 =-22.1.degree.). As a result,
it was surprisingly revealed that the pure (-)-n- isopulegol is odorless,
giving off no minty and herbaceous odor which has hitherto been considered
essential to isopulegol, but only possesses refreshing stimulation giving
a feeling of coolness.

In order to identify the perfuming components of the unpurified
(-)-n-isopulegol, the mother liquor was subjected to precise fractional
distillation (rectification) on a 40-plate Heli-Pack distillation tower.
Gas chromatography of the distillate lent confirmation to the presence of
impurities which are by-produced in cyclization of citronellal, such as
3,8-paramenthadiene, 2,8-paramenthadiene, 3-methylcyclohexanol, menthone,
and isomenthone, revealing that these compounds are the main cause of what
has been called fragrance of isopulegol.

Since the above-mentioned purification method consisting of
recrystallization many times is costly for industrial application, the
inventor has studied an economical method for synthesizing chemically and
optically pure odorless (-)-n-isopulegol.

First of all, rectification of (-)-n-isopulegol having an optical purity of
97.5%e.e. and a chemical purity of 97.5% by weight was attempted by using
a distillation tower, such as a Heli-Pack column having 100 theoretical
plates. It was confirmed that the thus purified (-)-n- isopulegol has a
reduced and yet perceivable odor characteristic of conventionally
available isopulegol and that mere distillation does not render isopulegol
odorless.

As a next approach, the inventor attempted recrystallization using a
variety of solvents for deep cooling and found acetone especially
excellent as a deep cooling solvent as demonstrated in Example 3
hereinafter described.

Thus, it was found that deep cooling using acetone as a solvent affords
needle-shaped particulate crystals having a high bulk density which neatly
settle to the bottom of a reaction container without adhering to the
container or a stirrer. If, on the other hand, other solvents like
petroleum ether are used as a solvent, it turned out that the precipitated
crystals are light and fluffy needle-like crystals, which easily adhere to
the reaction container or a stirrer only to attain a poor separation
efficiency in the subsequent centrifugation. Therefore, recrystallization
using these solvents is not efficient for removing a trace amount of the
impurity.

Since the crystals obtained by a single deep cooling operation according to
the present invention have high purity both chemically and optically, a
final product ((-)-n-isopulegol) obtained therefrom simply by centrifugal
separation followed by cutting the initial fraction by means of a
distillation tower is optically and chemically pure as analyzed by gas
chromatography, having an optical purity as high as 99.7%e.e. or even
higher and a chemical purity as high as 99.7% by weight or even higher,
and has no odor but pleasant refreshing stimulation.

When deep cooling is conducted using an oxygen-containing compound, such as
ethyl acetate, methanol, ethanol, tetrahydrofuran, methyl ethyl ketone,
dipropyl ether, and diethyl ether, as a solvent, the purity of isopulegol
can be increased slightly. Acetone is the most preferred solvent.

Acetone is preferably used in an amount 1 to 5 times, still preferably 1.5
to 3.0 times, in terms of volume per weight, as much as the isopulegol.

A minor proportion (preferably at most 30% by weight) of acetone may be
replaced with the aforesaid oxygen-containing solvent.

The deep cooling temperature preferably ranges from -20.degree. to
-60.degree. C., still preferably from -25.degree. to -50.degree. C.

After the deep cooling, the crystals are collected by centrifugation and
then subjected to rectification with, for example, a 5 to 40-plate
Heli-Pack distillation tower to obtain a final product for the market.
Where isopulegol to be purified is subjected to rectification before deep
cooling, the crystals may be subjected to single distillation to obtain a
final product.

The thus obtained odorless and chemically and optically pure
(-)-n-isopulegol having a chemical and optical purity of not lower than
99.7% can be used to impart a feeling of freshness, crispness, and
coolness to a citrus fragrance.

The citrus fragrance which can be used in the present invention includes
those of lemons, oranges, mandarins, litchis, bergamot, and grapefruits.

(-)-n-Isopulegol according to the present invention is used in a citrus
perfume composition in a proportion of 3 to 30% by weight, preferably 5 to
20% by weight. If the proportion is less than 3% by weight, the perfume
composition tends to lack freshness. If it exceeds 30% by weight, the top
note of the composition tends to be unbalanced.

The present invention is now illustrated in greater detail with reference
to Examples, but it should be understood that the present invention is not
limited thereto.

EXAMPLE 1

Purification of (-)-n-Isopulegol

(-)-n-Isopulegol which is an intermediate for the synthesis of menthol was
obtained by the above-described Takasago Process. The intermediate has a
chemical purity of 97.1% ›impurity composition: 0.9% iso-form, 1.7%
neo-form, not more than 0.1% neoiso-form, and not more than 0.1% others
(3,8-paramenthadiene and other several compounds)! and an optical purity
of 97.5%e.e. (›.alpha.!.sub.D.sup.25 =-20.90.degree.) as analyzed by gas
chromatography (GC) under the following conditions and has a herbaceous
minty fragrance.

Temperature: elevated at a rate of 2.degree. C./min from 60.degree. C. up
to a constant temperature of 190.degree.

Retention time for (+)-n-compound: 27.9 min

Retention time for (-)-n-compound: 28.3 min

In a 3 l-volume 4-necked container for deep cooling was put 500 g of the
above-mentioned (-)-n-isopulegol, and 1500 ml of acetone was added
thereto. The mixture was cooled to -40.degree. C. in a nitrogen stream,
and the precipitated solid was separated by centrifugation to obtain 367 g
of crystals.

The crystals were melted and distilled in a 40-plate Heli-Pack tower to
obtain 305 g of (-)-n-isopulegol (64.5.degree. C./1 mmHg) which was found
to be 100% pure both optically and chemically (›.alpha.!.sub.D.sup.25
=-22.1.degree.) as analyzed by gas chromatography. The crystals were
odorless and had a refreshing feeling of coolness.

EXAMPLE 2

Purification of (-)-n-Isopuleqol

(-)-n-Isopulegol having a purity of 69% (as measured by gas chromatography)
was synthesized from (+)-citronellal of citronella oil origin (optical
purity: 81%e.e.) by the process of using silica gel (U.S. Pat. No.
3,218,361). In a 3 l-volume 4-necked container for deep cooling was put
800 g of the (-)-n-isopulegol in a nitrogen stream, 1,500 ml of n-heptane
was added to the container, and the mixture was cooled to -45.degree. C.
The precipitated crystals were separated by centrifugation to collect 480
g of crystals having a chemical purity of 94% and an optical purity of
95%e.e.

The crystals (450 g) were melted and placed in a 2 l-volume 4-necked
container for deep cooling in a nitrogen stream, and 1200 ml of acetone
was added thereto. The mixture was cooled to -40.degree. C. in a nitrogen
stream, and the precipitated solid was separated by centrifugation to
obtain 304 g of crystals having a chemical purity of 99.6% and an optical
purity of 99.9%e.e.

The crystals (300 g) were melted and put in a 1 l-volume 4-necked container
for deep cooling in a nitrogen stream, and 400 ml of acetone was added
thereto. The mixture was cooled to -35.degree. C. in a nitrogen stream,
and the precipitated solid was separated by centrifugation to obtain 233 g
of crystals having a chemical purity of 100% and an optical purity of
100%e.e.

The resulting crystals (220 g) were melted and distilled in a Widmer spiral
to obtain 190 g of (-)-n- isopulegol (64.5.degree. C./1 mmHg) which was
found to be 100% pure both optically and chemically
(›.alpha.!).sub.D.sup.25 =-22.1.degree.) as analyzed by gas
chromatography. The crystals were odorless and had a refreshing feeling of
coolness.

EXAMPLE 3

(-)-n-Isopulegol was purified in the same manner as in Example 1, except
for changing acetone with various other solvents. The physical properties
of the (-)-n-isopulegol crystals obtained by deep cooling are shown in
Table 1 below. Further, the odor of the resulting (-)-n-isopulegol
crystals was organoleptically evaluated by 30 panel members.

As a result of organoleptic evaluation by 30 panel members, all the panel
members judged that Formulation 2 had its fragrance unbalanced by the odor
of the conventional (-)-n-isopulegol and that Formulation 1 containing no
isopulegol was much better. Further, they all preferred Formulation 3
containing the (-)-n-isopulegol obtained in Example 1 to Formulation 1,
indicating that the former had enhanced freshness with a character closer
to a natural fresh lime.

EXAMPLE 5

Formulation of Citrus Fragrance

A fresh neroli type cologne which suits all tastes was prepared according
to Formulation 4 shown below. The (-)-n-isopulegol content in the
composition was 10% by weight.

According to the present invention, (-)-n-isopulegol of extremely high
purity can be obtained by using acetone as a solvent for deep cooling.
Incorporation of the isopulegol obtained by the purification method of the
present invention which is odorless and has chemical and optical purity of
not less than 99.7% provides a citrus perfume composition having a feeling
of freshness, crispness, and coolness and meeting all the taste.

While the invention has been described in detail and with reference to
specific examples thereof, it will be apparent to one skilled in the art
that various changes and modifications can be made therein without
departing from the spirit and scope thereof.